In recent years, a magnetic random access memory (MRAM) has been developed as a memory using magnetic material. The MRAM uses, as an element device, a magnetic tunneling junction (MTJ) that utilizes a tunneling magnetoresistive (TMR) effect. The MTJ element has a structure of a non-magnet layer (insulating layer) sandwiched by two ferromagnet layers (a recording layer and a pinned layer), in which the magnetization direction of one of the ferromagnet layers (recording layer) can be reversed by an external magnetic field. Thus, in the MTJ element, information is recorded by controlling the magnetization direction of a magnet layer. Because the magnetization direction of the magnet layer does not change even when power supply is turned off, a non-volatile operation in which the recorded information is retained can be realized. The magnetization direction of the MTJ element can be changed (i.e., information can be rewritten) by, in addition to the system of applying a magnetic field from the outside, a spin transfer torque magnetization reversal (spin injection magnetization reversal) system that has recently been identified, by which the magnetization is reversed by causing a DC current to flow through the MTJ element directly. For example, Patent Document 1 discloses a MTJ element using the MTJ element with an in-plane magnetization easy axis (in-plane MTJ element) as the recording layer, and utilizing spin injection magnetization reversal, and a spin-transfer torque magnetic random access memory (SPRAM) which is a memory integrating the MTJ elements. The SPRAM may be referred to as a STT-MRAM.
The resistance of the MTJ element is varied by a difference in magnetization direction between the recording layer and the pinned layer. The ratio of change in resistance is referred to as a tunnel magnetoresistive (TMR) ratio. In memory applications, a high TMR ratio is desirable in order to read the information of “0” and “1” without error. In order to obtain a high TMR ratio, crystal orientation control of a tunnel barrier layer and high polarizability magnetic layers on both sides of the tunnel barrier layer is important. From the past studies on the in-plane MTJ, it is known that a high TMR ratio can be obtained when MgO (001) with a NaCl structure is used as the tunnel barrier layer, with CoFeB layer or CoFe layer with a bcc (001) crystal structure disposed on both sides of the tunnel barrier layer. When CoFeB is formed at room temperature, CoFeB grows amorphously. When MgO is formed thereon, a MgO (001) crystal grows. After CoFeB is formed further thereon, when an anneal process is performed, a CoFeB layer is crystal-oriented in bcc (001) with the MgO (001) crystal as a nucleus. In the case of an in-plane magnetization TMR element, MgO (001) and bcc (001) orientation of CoFeB are realized by using such a mechanism.
Further, in a SPRAM, current is caused to flow by a transistor connected to the MTJ element so as to reverse the magnetization of the recording layer of the MTJ element. When the gate length of the transistor is decreased as a result of an increase in memory integration, the amount of current that the transistor can cause to flow is also decreased. Thus, a lower write current Ic0 is required for the MTJ element used in the SRPAM. Further, when element miniaturization is attempted, thermal stability of magnetic information in the MTJ element presents an issue. When the thermal energy (kBT, where kB is the Boltzmann constant, and T is the absolute temperature) due to environment temperature is high with respect to the magnetic energy barrier (E) required for reversing the magnetization direction of the recording layer of the MTJ element, magnetization reversal occurs without application of an external magnetic field or current. Because the magnetic energy barrier of the MTJ element is decreased with decreasing size, the thermal stability factor E/kBT is reduced as a result of element miniaturization. Accordingly, the MTJ element applied in a SPRAM requires high TMR ratio and E/kBT, and a low write current Ic0.
As a promising structure for improving the high E/kBT and low Ic0 characteristics, development of an MTJ element using perpendicular magnetization material (perpendicular MTJ element) is underway (see Patent Document 2, for example). Also, a new perpendicular MTJ element structure that uses CoFeB as a perpendicular magnetization material has been identified (Non-Patent Document 1). Normally, CoFeB is a material that exhibits an in-plane magnetization easy axis. However, in a structure in which an oxide layer, such as MgO, is disposed on an interface of CoFeB, perpendicular magnetic anisotropy appears as the CoFeB film thickness is decreased.    Patent Document 1: JP 2005-116923 A    Patent Document 2: JP 2007-142364 A    Non-Patent Document 1: S. Ikeda et al., Nature Materials, 9, 721 (2010)